The light-scattering type smoke detector is an apparatus which comprises a dark chamber into which smoke can enter and wherein a light-emitting element and a light-receiving element are disposed at positions where the light beam from the light-emitting element does not directly impinge in the light-receiving element, and which by sensing scattering of light caused by the minute particles of any smoke which enters the dark chamber, generates a fire alarm or the like. There is known a smoke detector of this type which is adjusted so that the base level noise light (background) can be sensed, whereby the detector generates a malfunction signal when the base level noise light is not detected (Laid-Open Utility Model Publication No. 32183/84, Laid-Open Patent Publication No. 21511/84, etc.).
Generally, the inside surface of the dark chamber (usually of a labyrinth structure) which smoke can enter is matte black, because the lower noise light, the better. That is, the background level is very low.
Even with such low background, breakage of a light-emitting element, disconnection, etc. can satisfactorily be detected, but a condition such as deterioration of a light-emitting element caused by aging, etc. cannot easily be detected with such low background.
In this case, if the inside surface of the dark chamber is made light, that is, if the inside surface is brightened, the background (base level noise light) is increased, and therefore, deterioration in the performance of apparatus parts can more easily be detected. But, as the noise level is raised, the difference between signal and noise decreases and thus inaccuracy in alarm generation is invited.
We studied how output of the light-receiving element changes when lightness of the inside surface of the dark chamber is raised, and we found that if the Munsell value of lightness of the inside surface of the dark chamber is raised up to 2-5, detection of smoke is not substantially influenced contrary to the general conviction. No one has considered venturing to raise the lightness of the inside surface of the dark chamber.